There is surprisingly little quantitative information concerning age-related changes in gastrointestinal absorption. The available literature is fraught with errors in experimental design or data analysis, making it impossible to reach any valid conclusions about differences in absorption with age. Most of those errors are a result of a lack of appreciation for basic pharmacokinetic principles. The purpose of the research program described here is to examine potential age-related differences in the absorption of four selected test compounds. The test compounds selected include endogenous and exogenous substances absorbed by different processes including passive and specialized transport (xylose, riboflavin, ascorbic acid and aspirin). Absorption will be assessed based upon the urinary excretion of unchanged compound after oral and intravenous dosing on separate occasions. In the case of aspirin all forms of the compound in the urine will be qauntitated after an oral dose. The study has been designed to optimize the information which may be obtained and will include the determination of: elimination half-life, renal and total body clearances and volumes of distribution. Salicylic acid plasma protein binding will also be determined in all plasma samples. All of the above parameters will be examined as a function of age and correlation analyses performed.